1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using the multi-beam scanning optical system. The present invention can be suitably applied to, for example, a laser beam printer or a digital copy machine with which high-speed, high-quality printing can be performed by simultaneously forming (recording) a plurality of scanning lines (dots) on the surface of a photosensitive member.
2. Description of the Related Art
In image forming apparatuses such as laser beam printers and digital copy machines, multi-beam scanning optical systems, with which a plurality of scanning lines can be simultaneously formed on the surface of a photosensitive member, are commonly used in order to perform high-speed printing.
In multi-beam scanning optical systems, there is a problem in that if the oscillation wavelengths of light sources are different from each other, dots in scanning lines formed by laser beams emitted from the light sources are displaced in a main-scanning direction due to the chromatic aberration of scanning lenses.
Accordingly, in Japanese Unexamined Patent Application Publication No. 2000-111820, a system is disclosed in which relative differences between the oscillation wavelengths of light sources are limited so that the dot displacement is reduced to within one-half of the size of a single pixel.
However, in color-image forming apparatuses which output halftone images such as color laser printers, etc., the dot displacement is not small enough when the allowable value thereof is one-half of the size of a single pixel.
FIGS. 12 and 13 are diagrams showing an example of a halftone image pattern which is generally used for forming a color image, where FIG. 12 shows a case in which dots are arranged without displacements in the main-scanning direction and FIG. 13 shows a case in which the dots are displaced in the main-scanning direction.
FIGS. 12 and 13 show a case in which two light beams are used. The solid lines extending in the horizontal direction are formed by one of the two light beams, and the dashed lines extending in the horizontal direction are formed by the other one of the light beams. In addition, the circles shown on the horizontal lines are dots formed by the scanning lines.
In FIG. 13, a region in which the gaps between the dots are wide (Wa) and a region in which the gaps between the dots are narrow (Wb) are alternately formed in an inclined manner. Since the regions in which the gaps between the dots are narrow appear dark and the regions in which the gaps between the dots are wide appear light in an actual image, this image looks like a striped pattern over the entire image area. In color laser printers, etc., many kinds of patterns like that shown in FIGS. 12 and 13 are used, and a small dot displacement may greatly affect the image depending on the pattern. Therefore, it is necessary to set an allowable value of the dot displacement (Wa-Wb) to an extremely small value such as one-fourth of the size of a single pixel.
The above-described dot displacement in the main-scanning direction is caused not only by the difference in oscillation wavelengths but also by a difference in light path lengths.
Next, the dot displacement due to a difference in light path lengths will be described below with reference to FIGS. 14 and 15.
FIG. 14 is a sectional view of the main part of a multi-beam scanning optical system cut along the main-scanning direction (main-scanning sectional view), and FIG. 15 is a sectional view of the main part of the multi-beam scanning optical system shown in FIG. 14 cut along the sub-scanning direction (sub-scanning sectional view).
In FIGS. 14 and 15, a light source unit 100 includes two light sources (laser beam sources) 101 and 102 formed of, for example, semiconductor lasers. A collimator lens 103 collimates two laser beams emitted by the light source unit 100, and a cylindrical lens 104 has a predetermined refractive power only in the sub-scanning direction. In addition, an aperture diaphragm 108 forms the laser beams emitted from the cylindrical lens 104 into optimal shapes. The collimator lens 103, the cylindrical lens 104, and the aperture diaphragm 108 form one element of an incident optical unit 114.
A deflector 105 serves as a deflecting unit, and is formed of, for example, a rotating polygon mirror. The deflector 105 is rotated in the direction shown by the arrow A at a constant speed by a driving unit (not shown) such as a motor, etc. A scanning optical unit 106 has fxcex8 characteristics and includes first and second fxcex8 lenses 106a and 106b. The two laser beams deflected by the deflector 105 are focused onto the surface of a photosensitive member (recording medium) 107 by the scanning optical unit 106 in the shape of spots, so that two scanning lines S101 and S102 are formed. The scanning optical unit 106 is constructed such that a deflecting surface 105a of the deflector 105 and the surface of the photosensitive member 107 are conjugate to each other in the sub-scanning cross section, so that surface tilting is corrected.
The photosensitive member (photosensitive drum) 107 has an approximately cylindrical shape, and serves as a recording medium.
In FIGS. 14 and 15, two laser beams B101 and B102, which are optically modulated in accordance with image information, are emitted from the light source unit 100, collimated by the collimator lens 103, and incident on the cylindrical lens 104. The two laser beams B101 and B102 incident on the cylindrical lens 104 leave the cylindrical lens 104 without a change in the main-scanning cross section, and pass through the aperture diaphragm 108 (a part of each laser beam is blocked). In the sub-scanning cross section, the two laser beams B101 and B102 converge before they pass through the aperture diaphragm 108 (a part of each laser beam is blocked). Accordingly, the two laser beams B101 and B102 are focused onto the deflecting surface 105a of the deflector 105 in the shape of lines (lines that extend in the main-scanning direction). Then, the laser beams B101 and B102 are deflected by the deflecting surface 105a of the deflector 105 and focused onto the surface of the photosensitive member 107 by the scanning optical unit 106 in the shape of spots. By rotating the deflector 105 in the direction shown by the arrow A, the laser beams B101 and B102 scan over the surface of the photosensitive member 107 in the direction shown by the arrow B (in the main-scanning direction) at a constant speed. Accordingly, an image is recorded on the surface of the photosensitive member 107, which serves as the recording medium.
In FIGS. 14 and 15, the laser beams B101 and B102 are emitted from the light sources 101 and 102, travel along light paths L101 and L102, and form the scanning lines S101 and S102, respectively.
As shown in FIG. 15, the laser beams B101 and B102 must be incident on the surface of the photosensitive member 107 at positions displaced from the end point T of the photosensitive member 107 in the sub-scanning direction. In the case in which the laser beams B101 and B102 are incident on the surface of the photosensitive member 107 at the end point T, the following problem occurs. That is, when the laser beams B101 and B102 are at positions close to the central point in the main-scanning direction, they are reflected by the surface of the photosensitive member 107, travel along the same light paths along which they have traveled in the reverse direction, and return to the light sources 101 and 102. Accordingly, the optical outputs of the semiconductor lasers vary due to noise caused by the laser beams returning from the photosensitive member 107, and the density of a printed image also varies.
However, when the laser beams B101 and B102 are incident on the surface of the photosensitive member 107 at positions displaced from the end point T, the lengths of the light paths L101 and L102 differ by xcex94B. In this case, as is apparent from FIG. 15, the lengths of the two scanning lines S101 and S102 also differ from each other. This is the manner in which the dot displacement in the main-scanning direction occurs due to the difference in light path lengths. Moreover, the dot displacement in the main-scanning direction caused by the difference in light path lengths is not small enough to be ignored.
Accordingly, an object of the present invention is to provide a multi-beam scanning optical system and an image forming apparatus using the multi-beam scanning optical system, in which the dot displacement in the main-scanning direction caused by the difference in wavelengths of the light sources and that caused by the difference in light path lengths of the laser beams counterbalance each other. In this way, the difference between magnifications can be corrected and the dot displacement in the main-scanning direction can be reduced, so that high-quality printing can be performed.
According to a first aspect of the present invention, a multi-beam scanning optical system includes a plurality of light sources; a deflecting unit which deflects a plurality of laser beams emitted from the light sources; and a scanning optical unit which focuses the laser beams deflected by the deflecting unit onto the surface of a photosensitive member. The scanning optical unit is set such that lateral chromatic aberration is overcorrected. In addition, among angles formed between each laser beam incident on the surface of the photosensitive member and the normal at the surface of the photosensitive member in a sub-scanning direction, the oscillation wavelength of the light source that emits a laser beam forming the minimum angle is set to a value smaller than the oscillation wavelength of the light source that emits a laser beam forming the maximum angle.
According to a second aspect of the present invention, a multi-beam scanning optical system includes n light sources; a deflecting unit which deflects n laser beams emitted from the n light sources; and a scanning optical unit which focuses the n laser beams deflected by the deflecting unit onto the surface of a photosensitive member having a cylindrical shape and forms n scanning lines. The scanning optical unit is set such that lateral chromatic aberration is overcorrected. In addition, when the first scanning line is formed on the surface of the photosensitive member at a position displaced from an end point of the photosensitive member by a distance S in a sub-scanning direction and the mth (1 less than mxe2x89xa6n) scanning line is formed on the surface of the photosensitive member at a position displaced from the end point by a distance (S+d) in the sub-scanning direction, and when the oscillation wavelength of the light source which emits the light beam forming the first scanning line is defined as xcex1 and the oscillation wavelength of the light source which emits the light beam forming the mth scanning line is defined as xcexm, the following expressions are satisfied:             "LeftBracketingBar"                        Δ          ⁢                      xe2x80x83                    ⁢                      Y            1                          -                  Δ          ⁢                      xe2x80x83                    ⁢                                    Y              2                        ⁡                          (                                                λ                  m                                -                                  λ                  1                                            )                                          "RightBracketingBar"        ≤          D      4                  Δ      ⁢              xe2x80x83            ⁢              Y        1              =          Δ      ⁢              xe2x80x83            ⁢      L      ⁢              xe2x80x83            ⁢      tan      ⁢              xe2x80x83            ⁢      α                  Δ      ⁢              xe2x80x83            ⁢      L        =                                        R            2                    -                                    (                              S                +                d                            )                        2                              -                                    R            2                    -                      S            2                              
wherein,
xcex94Y2: dot displacement in the main-scanning direction at the end in the main-scanning direction caused per unit wavelength
D: size of a single pixel
xcex1: maximum angle among angles formed between the normal at the surface of the photosensitive member and the laser beams in the main-scanning direction
R: radius of the photosensitive member.
In this case, preferably, xcex1 and xcexm satisfy the following expression:
xe2x88x921xe2x89xa6xcexmxe2x88x92xcex1xe2x89xa63 (unit: nm).
In the multi-beam scanning optical system according to the above-described first and second aspects of the present invention, the scanning optical unit may include at least one diffractive optical element.
In addition, the multi-beam scanning optical system according to the above-described first and second aspects of the present invention may further include a synchronization position detection unit in which parts of the laser beams deflected by the deflecting unit are directed to a synchronization detection element by a synchronization detection lens, and which controls the time at scanning start position on the surface of the photosensitive member by using a signal obtained from the synchronization detection element. The synchronization detection lens is disposed in such a manner that the synchronization detection lens is centered and untilted relative to a light path from the deflecting unit to the synchronization detection element.
According to a third aspect of the present invention, a multi-beam scanning optical system includes a plurality of light sources; a deflecting unit which deflects a plurality of laser beams emitted from the light sources; and a scanning optical unit which focuses the laser beams deflected by the deflecting unit onto the surface of a photosensitive member. The scanning optical unit is set such that lateral chromatic aberration is undercorrected. In addition, among angles formed between each laser beam incident on the surface of the photosensitive member and the normal at the surface of the photosensitive member in a sub-scanning direction, the oscillation wavelength of the light source that emits a laser beam forming the minimum angle is set to a value larger than the oscillation wavelength of the light source that emits a laser beam forming the maximum angle.
According to a fourth aspect of the present invention, a multi-beam scanning optical system includes n light sources; a deflecting unit which deflects n laser beams emitted from the n light sources; and a scanning optical unit which focuses the n laser beams deflected by the deflecting unit onto the surface of a photosensitive member having a cylindrical shape and forms n scanning lines. The scanning optical unit is set such that lateral chromatic aberration is undercorrected. In addition, when the first scanning line is formed on the surface of the photosensitive member at a position displaced from an end point of the photosensitive member by a distance S in a sub-scanning direction and the mth (1 less than mxe2x89xa6n) scanning line is formed on the surface of the photosensitive member at a position displaced from the end point by a distance (S+d) in the sub-scanning direction, and when the oscillation wavelength of the light source which emits the light beam forming the first scanning line is defined as xcex1 and the oscillation wavelength of the light source which emits the light beam forming the mth scanning line is defined as xcexm, the following expressions are satisfied:             "LeftBracketingBar"                        Δ          ⁢                      xe2x80x83                    ⁢                      Y            1                          -                  Δ          ⁢                      xe2x80x83                    ⁢                                    Y              2                        ⁡                          (                                                λ                  1                                -                                  λ                  m                                            )                                          "RightBracketingBar"        ≤          D      4                  Δ      ⁢              xe2x80x83            ⁢              Y        1              =          Δ      ⁢              xe2x80x83            ⁢      L      ⁢              xe2x80x83            ⁢      tan      ⁢              xe2x80x83            ⁢      α                  Δ      ⁢              xe2x80x83            ⁢      L        =                                        R            2                    -                                    (                              S                +                d                            )                        2                              -                                    R            2                    -                      S            2                              
wherein,
xcex94Y2: dot displacement in the main-scanning direction at the end in the main-scanning direction caused per unit wavelength
D: size of a single pixel
xcex1: maximum angle among angles formed between the normal at the surface of the photosensitive member and the laser beams in the main-scanning direction
R: radius of the photosensitive member.
In this case, preferably, xcex1 and xcexm satisfy the following expression:
xe2x88x921xe2x89xa6xcex1xe2x88x92xcexmxe2x89xa63 (unit: nm).
In the multi-beam scanning optical system according to the above-described third and fourth aspect of the present invention, the scanning optical unit may include at least one diffractive optical element.
In addition, the multi-beam scanning optical system according to the above-described third and fourth aspect of the present invention may further include a synchronization position detection unit in which parts of the laser beams deflected by the deflecting unit are directed to a synchronization detection element by a synchronization detection lens, and which controls the time at scanning start position on the surface of the photosensitive member by using a signal obtained from the synchronization detection element. The synchronization detection lens is disposed in such a manner that the synchronization detection lens is centered and untilted relative to a light path from the deflecting unit to the synchronization detection element.
According to a fifth aspect of the present invention, a multi-beam scanning optical system includes a plurality of light sources; a deflecting unit which deflects a plurality of laser beams emitted from the light sources; and a scanning optical unit which focuses the laser beams deflected by the deflecting unit onto the surface of a photosensitive member. A dot displacement direction in a main-scanning direction due to a difference in light path lengths of the laser beams is opposite to a dot displacement direction in the main-scanning direction due to a difference in wavelengths of the laser beams.
In this case, preferably, the dot displacement in the main-scanning direction due to the difference in light path lengths of the laser beams and the dot displacement in the main-scanning direction due to the difference in wavelengths of the laser beams counterbalance each other.
In addition, according to the present invention, an image forming apparatus includes the multi-beam scanning optical system of the present invention; a photosensitive member disposed on a scan surface; a developing unit which develops an electrostatic latent image formed on the photosensitive member as a toner image, the electrostatic latent image being formed by the laser beams which are emitted from the multi-beam scanning optical system and which scan over the photosensitive member; a transfer unit which transfers the toner image developed by the developing unit onto a transfer medium; and a fixing unit which fixes the toner image transferred by the transfer unit on the transfer medium.
In addition, according to the present invention, an image forming apparatus includes the multi-beam scanning optical system of the present invention; and a printer controller which converts code data obtained from an external device into an image signal and inputs the image signal to the multi-beam scanning optical system.
Thus, according to the present invention, the dot displacement in the main-scanning direction due to the difference in light path lengths of the laser beams and the dot displacement in the main-scanning direction due to the difference in wavelengths of the light sources counterbalance each other, so that the difference in overall magnifications can be corrected. Accordingly, a multi-beam scanning optical system which performs high-quality printing by reducing the dot displacement in the main-scanning direction and an image forming apparatus using the multi-beam scanning optical system can be provided.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.